In the fabrication of light-emitting diodes, III-nitride-based semiconductors, such as GaN, AlGaN, InGaN and AlInGaN, are common. Typically, epitaxial structures of most of the light-emitting devices made of the III-nitride-based semiconductors are grown on an electrically insulating sapphire substrate, which is different from other light-emitting devices utilizing conductive substrates. The sapphire substrate is an insulator, so an electrode cannot be directly formed on the sapphire substrate. Electrodes have to be formed to contact respectively a p-type semiconductor layer and an n-type semiconductor layer directly, so that the light-emitting devices of the aforementioned type can be completed.
FIGS. 1a through 1d are schematic flow diagrams showing the process for manufacturing a conventional light-emitting diode. A substrate 100 made of sapphire is provided, and then an n-type semiconductor layer 102, an active layer 104, and a p-type semiconductor layer 106 are grown on the substrate 100 in sequence by an epitaxial method. The n-type semiconductor layer 102, the active layer 104 and the p-type semiconductor layer 106 comprise an illuminant epitaxial structure 108, such as shown in FIG. 1a. 
An electrically conductible substrate 110 is provided, and an adhesion layer 112 is formed on a surface of the substrate 110, such as shown in FIG. 1b. Next, a wafer bonding step is performed to adhere the substrate 110 to the p-type semiconductor layer 106 by the adhesion layer 112 for achieving the connection of the substrate 110 and the illuminant epitaxial structure 108, such as shown in FIG. 1c. After the substrate 110 and the illuminant epitaxial structure 108 are bonded, the electrically insulating substrate 100 is removed to complete the manufacturing of a light-emitting diode 114, such as shown in FIG. 1d. 
However, in such a bonding technique, bubbles are produced at the bonding interface between the illuminant epitaxial structure 108 and the adhesion layer 112 when the bonding step is performed by using the adhesion layer 112. The bubbles greatly degrade the bonding strength. Due to the insufficient bonding strength, the substrate 110 very easily falls off the illuminant epitaxial structure 108 or chip, thereby degrading the yield of the sequent processes.